Air calibration scan for computed tomography scanner with obstructing objects

ABSTRACT

A method and apparatus for performing CT scans of baggage being carried or loaded onto commercial aircraft are described. The CT baggage scanner of the invention includes numerous features which provide the system with high baggage throughput on the order of seven hundred bags per hour as well as improved image quality and accurate target detection. In one aspect, the scanner includes an adaptive image reconstruction window which identifies data collected from the field of view that are not related to the baggage being scanned. These unrelated data are excluded from the image reconstruction process, resulting in greatly reduced reconstruction time and increased baggage throughput. The invention also includes the capability of performing calibration &#34;air scans&#34; with objects such as the system conveyor in the field of view. Data gathered during the calibration scan are applied to a threshold, and data exceeding the threshold are assumed to be from X-rays that are unobstructed by objects in the field of view and are therefore used to perform the air calibration. The baggage scanner can also analyze scan data to identify shapes of objects, particularly, objects formed in the shape of a sheet. This greatly improves the ability of the system to detect sheet explosives. The system also compensates for detector dark currents and provides dark current offsets which can be dependent upon detector temperature.

RELATED APPLICATION

This application is related to copending U.S. application Ser. No.08/831,558, filed on Apr. 9, 1997, (Attorney Docket No. ANA-118) ofcommon assignee, the contents of which are incorporated herein in theirentirety by reference.

This application is related to the following U.S. applications filed oneven date herewith, of common assignee, the contents of which areincorporated herein in their entirety by reference:

"Computed Tomography Scanner Drive System and Bearing," invented byAndrew P. Tybinkowski, et al., (Attorney Docket No. ANA-128) U.S. Ser.No. 08/948,930;

"Computed Tomography Scanning Apparatus and Method With TemperatureCompensation for Dark Current Offsets," invented by Christopher C. Ruth,et al., (Attorney Docket No. ANA-131) U.S. Ser. No. 08/948,928;

"Computed Tomography Scanning Target Detection Using Non-ParallelSlices," invented by Christopher C. Ruth, et al., (Attorney Docket No.ANA-132) U.S. Ser. No. 08/948,491;

"Computed Tomography Scanning Target Detection Using Target SurfaceNormals," invented by Christopher C. Ruth, et al., (Attorney Docket No.ANA-133) U.S. Ser. No. 08/948 929;

"Parallel Processing Architecture for Computed Tomography ScanningSystem Using Non-Parallel Slices," invented by Christopher C. Ruth, etal., (Attorney Docket No. ANA-134) U.S. Ser. No. 08/948,697;

"Computed Tomography Scanning Apparatus and Method For GeneratingParallel Projections Using Non-Parallel Slice Data," invented byChristopher C. Ruth, et al., (Attorney Docket No. ANA-135) U.S. Ser. No.08/948,492;

"Computed Tomography Scanning Apparatus and Method Using AdaptiveReconstruction Window," invented by Bernard M. Gordon, et al., (AttorneyDocket No. ANA-136) U.S. Ser. No. 08/949 127;

"Area Detector Array for Computed Tomography Scanning System," inventedby David A Schafer, et al., (Attorney Docket No. ANA-137) U.S. Ser. No.08/948,450;

"Closed Loop Air Conditioning System for a Computed Tomography Scanner,"invented by Eric Bailey, et al., (Attorney Docket No. ANA-138) U.S. Ser.No. 08/948,692;

"Measurement and Control System for Controlling System Functions as aFunction of Rotational Parameters of a Rotating Device," invented byGeoffrey A. Legg, et al., (Attorney Docket No. ANA-139) U.S. Ser. No.08/948,698;

"Rotary Energy Shield for Computed Tomography Scanner," invented byAndrew P. Tybinkowski, et al., (Attorney Docket No. ANA-144) U.S. Ser.No. 08/948,698.

FIELD OF THE INVENTION

The present invention relates generally to computed tomography (CT)scanners and more specifically to a baggage scanning system whichutilizes CT technology.

BACKGROUND OF THE INVENTION

Various X-ray baggage scanning systems are known for detecting thepresence of explosives and other prohibited items in baggage, orluggage, prior to loading the baggage onto a commercial aircraft. Acommon technique of measuring a material's density is to expose thematerial to X-rays and to measure the amount of radiation absorbed bythe material, the absorption being indicative of the density. Since manyexplosive materials may be characterized by a range of densitiesdifferentiable from that of other items typically found in baggage,explosives are generally amenable to detection by X-ray equipment.

Most X-ray baggage scanning systems in use today are of the "linescanner" type and include a stationary X-ray source, a stationary lineardetector array, and a conveyor belt for transporting baggage between thesource and detector array as the baggage passes through the scanner. TheX-ray source generates an X-ray beam that passes through and ispartially attenuated by the baggage and is then received by the detectorarray. During each measuring interval the detector array generates datarepresentative of the integral of density of the planar segment of thebaggage through which the X-ray beam passes, and this data is used toform one or more raster lines of a two-dimensional image. As theconveyor belt transports the baggage past the stationary source anddetector array, the scanner generates a two-dimensional imagerepresentative of the density of the baggage, as viewed by thestationary detector array. The density image is typically displayed foranalysis by a human operator.

Techniques using dual energy X-ray sources are known for providingadditional information about a material's chemical characteristics,beyond solely a density measurement. Techniques using dual energy X-raysources involve measuring the X-ray absorption characteristics of amaterial for two different energy levels of X-rays. These measurementsprovide an indication of the material's atomic number in addition to anindication of the material's density. Dual energy X-ray techniques forenergy-selective reconstruction of X-ray CT images are described, forexample, in Alvarez, Robin et al., "Energy-selective Reconstructions inX-ray Computerized Tomography", Phys. Med. Biol. 1976, Vol. 21, No. 5,733-744; and U.S. Pat. No. 5,132,998.

One proposed use for such dual energy techniques has been in connectionwith a baggage scanner for detecting the presence of explosives inbaggage. Explosive materials are generally characterized by a knownrange of atomic numbers and are therefore amenable to detection by suchdual energy X-ray sources. One such dual energy source is described incopending U.S. patent application Ser. No. 08/671,202, entitled"Improved Dual Energy Power Supply," (Attorney Docket No. ANA-094) whichis assigned to the same assignee as the present invention and which isincorporated herein in its entirety by reference.

Plastic explosives present a particular challenge to baggage scanningsystems because, due to their moldable nature, plastic explosives may beformed into geometric shapes that are difficult to detect. Mostexplosives capable of significantly damaging an aircraft weigh at leasta pound and are sufficiently large in length, width, and height so as tobe readily detectable by an X-ray scanner system regardless of theexplosive's orientation within the baggage. However, a plastic explosivepowerful enough to damage an aircraft may be formed into a relativelythin sheet that is extremely small in one dimension and is relativelylarge in the other two dimensions. The detection of plastic explosivesmay be difficult because it may be difficult to see the explosivematerial in the image, particularly when the material is disposed sothat the thin sheet is parallel to the direction of the X-ray beam asthe sheet passes through the system.

Thus, detection of suspected baggage requires very attentive operators.The requirement for such attentiveness can result in greater operatorfatigue, and fatigue as well as any distractions can result in asuspected bag passing through the system undetected.

Accordingly, a great deal of effort has been made to design a betterbaggage scanner. Such designs, for example, have been described in U.S.Pat. Nos. 4,759,047 (Donges et al.); 4,884,289 (Glockmann et al.);5,132,988 (Tsutsui et al.); 5,182,764 (Peschmann et al.); 5,247,561(Kotowski); 5,319,547 (Krug et al.); 5,367,552 (Peschmann et al.);5,490,218 (Krug et al.) and German Offenlegungsschrift DE 31 503 06 A1(Heimann GmbH).

At least one of these designs, described in U.S. Pat. Nos. 5,182,764(Peschmann et al.) and 5,367,552 (Peschmann et al.) (hereinafter the'764 and '552 patents), has been commercially developed and is referredto hereinafter as the "Invision Machine." The Invision Machine includesa CT scanner ofthe third generation type, which typically include anX-ray source and an X-ray detector system secured respectively todiametrically opposite sides of an annular-shaped platform or disk. Thedisk is rotatably mounted within a gantry support so that in operationthe disk continuously rotates about a rotation axis while X-rays passfrom the source through an object positioned within the opening of thedisk to the detector system.

The detector system can include a linear array of detectors disposed asa single row in the shape of a circular arc having a center of curvatureat the focal spot of the X-ray source, i.e., the point within the X-raysource from which the X-rays emanate. The X-ray source generates a fanshaped beam, or fan beam, of X-rays that emanates from the focal spot,passes through a planar imaging field, and is received by the detectors.The CT scanner includes a coordinate system defined by X-, Y- andZ-axes, wherein the axes intersect and are all normal to one another atthe center of rotation of the disk as the disk rotates about therotation axis. This center of rotation is commonly referred to as the"isocenter." The Z-axis is defined by the rotation axis and the X- andY-axes are defined by and lie within the planar imaging field. The fanbeam is thus defined as the volume of space defined between a pointsource, i.e., the focal spot, and the receiving surfaces of thedetectors of the detector array exposed to the X-ray beam. Because thedimension of the receiving surfaces of the linear array of detectors isrelatively small in the Z-axis direction the fan beam is relatively thinin that direction. Each detector generates an output signalrepresentative of the intensity of the X-rays incident on that detector.Since the X-rays are partially attenuated by all the mass in their path,the output signal generated by each detector is representative of thedensity of all the mass disposed in the imaging field between the X-raysource and that detector.

As the disk rotates, the detector array is periodically sampled, and foreach measuring interval each of the detectors in the detector arraygenerates an output signal representative of the density of a portion ofthe object being scanned during that interval. The collection of all ofthe output signals generated by all the detectors in a single row of thedetector array for any measuring interval is referred to as a"projection," and the angular orientation of the disk (and thecorresponding angular orientations of the X-ray source and the detectorarray) during generation of a projection is referred to as the"projection angle." At each projection angle, the path of the X-raysfrom the focal spot to each detector, called a "ray," increases in crosssection from a point source to the receiving surface area of thedetector, and thus is thought to magnify the density measurement becausethe receiving surface area of the detector area is larger than any crosssectional area of the object through which the ray passes.

As the disk rotates around the object being scanned, the scannergenerates a plurality of projections at a corresponding plurality ofprojection angles. Using well known algorithms a CT image of the objectmay be generated from all the projection data collected at each of theprojection angles. The CT image is representative of the density of atwo dimensional "slice" of the object through which the fan beam haspassed during the rotation of the disk through the various projectionangles. The resolution of the CT image is determined in part by thewidth of the receiving surface area of each detector in the plane of thefan beam, the width of the detector being defined herein as thedimension measured in the same direction as the width of the fan beam,while the length of the detector is defined herein as the dimensionmeasured in a direction normal to the fan beam parallel to the rotationor Z-axis of the scanner.

One important design criterion for a baggage scanner is the speed withwhich the scanner can scan an item of baggage. To be of practicalutility in any major airport, a baggage scanner should be capable ofscanning a large number of bags at a very fast rate, e.g., on the orderof seven hundred of bags per hour or faster, and to provide this ratethe scanner must scan an average sized bag at a rate of about 5 secondsper bag or less. For this reason one problem with the Invision Machineis that CT scanners of the type described in the '764 and '552 patentstake a relatively long time, e.g., from about 0.6 to about 2.0 secondsfor one revolution of the disk, to generate the data for a single slicedCT image. Further, the thinner the slice of the beam through the bag foreach image the better the resolution of the image, so the CT scannershould provide images of sufficient resolution to detect plasticexplosives on the order of only a few millimeters thick. If 0.6 to 2.0seconds are required for generation of data for each sliced CT image,and the average bag can be assumed to be about 70cm long, at the desiredthroughput rate of 700 bags per hour a conventional CT baggage scannercan only afford to generate an average of two or three CT images per bagsince the bag must be moved and stopped at each location of a scan.Clearly, one cannot scan the entire bag within the time allotted for areasonably fast throughput. Generating only two or three CT images perbaggage item leaves most of the item unscanned and therefore does notprovide adequate or complete scanning.

SUMMARY OF THE INVENTION

The present invention is directed to a baggage scanning system whichsubstantially overcomes the drawbacks of the prior art. The baggagescanning system of the invention is capable of scanning on the order ofseven hundred bags per hour without the need for operator interventionas the baggage is transported through the scanner.

In one aspect, the present invention is directed to an apparatus andmethod in a CT scanner which greatly improve scanner throughput byadapting the data reconstruction window of the scanner to the size ofeach object, e.g., piece of baggage, being scanned. In this aspect, theinvention tailors the reconstruction window, which defines the number ofpixels to be reconstructed from scan data to generate an image, to thesize and location of the bag within the field of view of the scanner.The CT machine scans the field of view to generate the scan data for theobject passing through the scanner. The size of the object and itslocation within the field of view are determined. Using the size andlocation of the object, two portions of pixels in the field of view areidentified. The first portion of pixels is reconstructed to generate animage of the object, and the second identified portion of pixels is notreconstructed.

Hence, in this aspect of the invention, only pixels that provideinformation related to the bag being scanned, i.e., the first portion ofpixels, are processed during image reconstruction. Pixels that are notrelated to the bag, i.e., the second portion of pixels, are notreconstructed. Unrelated pixels can include those in the area below thebag conveyor system and in the areas next to and above the bag. Byeffectively discarding pixels that provide no information related to thebag, the processing load during reconstruction is substantially reduced,resulting in shorter reconstruction time and higher bag scanningthroughput.

In one embodiment, the size and location of the bag within the field ofview are determined by detecting the boundaries of the bag. This can bedone by analyzing the scan data to locate object boundaries in the datausing boundary location processes known in the art. In one embodiment,parallel projection data can be analyzed to locate the boundaries. In analternative embodiment, the scanner includes a separate sensor used todetect the bag boundaries. The sensor can be an acoustic sensor such asa high-frequency ultrasound range finder, or it can be an opticalsensor, which can include one or more optical devices such as lasers,light emitting diodes, or infrared detectors. Any of these approachesreturn data which indicate the boundaries of the bag, which can be usedto indicate the center of the bag and, therefore, its location withinthe scanner field of view.

In one embodiment, the pixels identified as being unrelated to the bagare not reconstructed, and all pixels related to the bag arereconstructed to generate a complete image of the bag. This approachprovides a complete image of every bag regardless of its size, andenjoys improved scanner throughput. However, because of the widevariation in bag sizes, the processing load and, therefore, the bagthroughput are difficult to monitor and control. There may existscenarios in which bag throughput could be reduced, such as where thescanner processes a large number of unusually large bags consecutively.Therefore, in an alternative embodiment, an upper limit is set on theoverall size of the reconstruction window. This can be done by setting amaximum number of pixels to be reconstructed. This maximum pixel windowcan then be fit within the determined size, location and dimensions ofthe bag to produce the best possible image of the bag within the presetpixel reconstruction limit. This approach produces a useful image of thebag while ensuring that the bag scanning throughput remains at acontrollable level.

By omitting pixels unrelated to the bag being analyzed from the imagereconstruction process, the adaptive reconstruction window of theinvention provides significant advantages. For example, significantunnecessary data processing is eliminated, resulting in shorterreconstruction time and increased bag throughput in accordance with thebag throughput required at busy commercial airports.

In another aspect, the invention is directed to a system and method forperforming calibration or "air" scans in a CT system to calibrate thesystem for variations in individual detector responses. Because it isgenerally difficult to remove obstructions, such as the conveyor system,from the field of view of the baggage scanner, air scans cannot readilybe performed in the same manner that air scans are performed onconventional CT machines. The scanner of the invention performs aircalibration scans while compensating for obstructions present in thefield of view.

In one embodiment, the calibration is performed by first performing ascan of the field of view and acquiring a full set of data withobstructions present in the field of view. A calibration threshold isset, and for each detector, view data that exceed the threshold areselected to be used in computing the calibration offset value for thedetector. Values below the threshold are discarded. In one embodiment,the threshold is set high enough such that it can be concluded that anydata values that exceed the threshold are for radiation rays that do notpass through obstructions in the field of view and, therefore, canappropriately be used for the air calibration. For each detector, theselected data values from the unobstructed views are used to compute anair calibration value for the detector. In one embodiment, the selectedvalues are averaged to compute the air calibration value. The aircalibration value is then used as a normalization during subsequentscans of actual objects to compensate for response variations fromdetector to detector.

By applying the air scan calibration data to a threshold, dataassociated with unobstructed ray paths are identified. This allows aircalibration to be performed without removing obstructions from the fieldof view. This is very important in the baggage scanner in which scansare performed continuously, and, as a result, it would be extremelyinconvenient to remove obstructions such as the conveyor from the fieldof view to perform an air scan between bags. However, this approach canalso be advantageous in the medical CT field. While it is somewhat moreconvenient to remove a patient table from a medical CT scanning machinethan it is to remove the conveyor from the baggage scanner, performingmedical CT air scans without the requirement of removing the patienttable would be a significant improvement in medical CT. Hence, the aircalibration of the invention also provides advantages in the medical CTfield.

In another aspect, the invention is directed to a method and apparatusfor identifying a target object such as a sheet from CT image data of anobject in three-dimensional space. It is well known that plasticexplosives can be molded into the shape of a sheet. Such sheetexplosives can be difficult to detect using conventional CT techniquessince the thickness of a sheet of explosives can be smaller than theresolution of a conventional CT scanner. In the present invention, imagedata for an object can be analyzed to determine if the object is in theform of a sheet and, therefore, could possibly be a plastic explosive.

In the apparatus and method of the invention, an object can be analyzedto determine if it is a sheet by analyzing image data near the surfaceof the object. The object to be analyzed is defined in three-dimensionalspace by its boundaries or surface. At each of many points along thesurface of the object, a surface normal is computed and projected backinto the object. At a plurality of points along the line projected backfrom the surface normal, an object density is obtained form the CT datafor the object. Interpolation can be used to compute data for eachpoint. A maximum distance into the object is set, and densities aregenerated up to the maximum distance. The maximum distance is chosen tobe larger than the maximum expected sheet thickness.

For each normal line, after all densities are generated, a distance intothe object at which the density rolls off is generated. In general, ifthe density rolls off at a distance less than the maximum distance, thedensity measurements may indicate the presence of a thin object whichmay be a sheet. Where no appreciable roll-off occurs up to the maximumdistance, an object thicker than a sheet is indicated.

The computed roll-off distances can be compiled in a distribution suchas a histogram. The histogram can then be analyzed to determine theshape of the object. A peak in the histogram at a roll-off distance lessthan the maximum distance can indicate a substantial portion of theobject having a thickness at that roll-off distance. This can be used toindicate a sheet. A high peak in the histogram at the maximum distancecan indicate a substantial portion of the object having a thicknesslarger than the expected thickness of a sheet. This can be used toindicate that the object is not a sheet.

The use of surface normals and histograms to identify a sheet-shapedobject provides advantages over other approaches to identify sheets. Forexample, in one prior approach, an object's shape is analyzed bycomputing the ratio of its surface area to its volume. A high ratio isused to indicate a thin object such as a sheet. However, this approachis not very precise in that it only computes a single number for anentire object, with that number being susceptible to interpretation.Some objects with large surface areas but relatively small volumes couldbe erroneously indicated as being sheet-shaped. In contrast, the presentinvention allows for analysis at points around the entire object. Byanalyzing a statistical distribution of thicknesses over the entireobject, a more precise conclusion as to the object's shape is obtained.

In another aspect, the invention is directed to an apparatus and methodfor providing compensation for "dark currents" in a CT system, i.e.,currents generated by the detectors in the absence of x-rays, andspecifically for compensating for variations in detector dark currentswith temperature. In accordance with the invention, a calibrationprocedure is performed to characterize the variation in the darkcurrents with temperature. Using this variation, a set of detectoroffsets is generated. Each offset defines the dark current or offsetcurrent for a particular temperature. In one embodiment, a set ofoffsets is generated for each detector. In another embodiment, one setof offsets is used for all detectors. During subsequent actual scanningof an object or region, the offsets are used to adjust the data signalsgenerated by the detectors. The temperature of the detectors is sensedwhile the region is scanned. For each detector, the offset associatedwith the presently sensed temperature is applied to the signal generatedby the detector to adjust the object density sensed by the detector andthereby compensate for the detector's dark current over temperature.

In one embodiment, the variation in temperature is characterized duringcalibration by fitting the offset-versus-temperature data points to aset of parametric equations. In one embodiment, the variation isdescribed by a Taylor series polynomial with constant coefficients. Thecoefficients can be derived by applying least squares error analysis.

By applying temperature dependent offsets to the detector data, thebaggage scanning system of the invention provides more accurate darkcurrent compensation than prior systems. In the baggage scanningenvironment, temperature effects are more substantial than they are inmedical settings, since the baggage scanning system runs continuously inmost cases. Therefore, the temperature dependency of the offsets becomesimportant to maintaining the quality of images created and,consequently, the ability of the system to detect target items. Thetemperature dependent offsets of the invention therefore provide a moreaccurate CT baggage scanner.

BRIEF DESCRIPTION OF THE DRAWINGS

The foregoing and other objects, features and advantages of theinvention will be apparent from the following more particulardescription of preferred embodiments of the invention, as illustrated inthe accompanying drawings in which like reference characters refer tothe same parts throughout the different views. The drawings are notnecessarily to scale emphasis instead being placed upon illustrating theprinciples of the invention.

FIG. 1 contains a perspective view of a baggage scanning system inaccordance with the present invention.

FIG. 2 contains a cross-sectional end view of the system shown in FIG.1.

FIG. 3 contains a cross-sectional radial view of the system shown inFIG. 1.

FIG. 4 contains a schematic electrical and mechanical block diagram ofone embodiment of the baggage scanner of the invention.

FIG. 5 is a schematic pictorial diagram of the field of view of thebaggage scanner of the invention showing a bag located on the conveyorsystem within the field of view.

FIG. 6 is a schematic plot illustrating the field of view of the baggagescanner of the invention superimposed on a Cartesian coordinate system.

FIG. 7 is a simplified schematic block diagram of one embodiment of thebaggage scanning system of the invention using sensors to identifyboundaries of a bag.

FIG. 8A is a schematic illustration of the geometric configuration ofthe source, detector array and field of view of a conventional CTscanner.

FIG. 8B is a schematic plot of a data signal obtained for a singledetector during a scan of the field of view shown in FIG. 8A.

FIG. 9A is a schematic illustration of the geometric configuration ofone embodiment of the baggage scanner of the present invention.

FIG. 9B is a schematic plot of a data signal obtained for a singledetector during a scan of the field of view of FIG. 9A.

FIG. 10 is a schematic illustration of a three-dimensional CT image of athree-dimensional object.

FIG. 11 is a schematic plot of density distributions along surfacenormal lines of CT images of a thin object and a thick object.

FIG. 12A is a schematic plot of a histogram of density roll-offdistances for a thin object.

FIG. 12B is a schematic plot of a histogram of density roll-offdistances for a thick object.

DETAILED DESCRIPTION OF THE DRAWINGS

FIGS. 1, 2 and 3 show perspective, end cross-sectional and radialcross-sectional views, respectively, of a baggage scanning system 100constructed according to the invention which provides improved abilityto detect the presence of target materials such as sheet explosivesregardless of their orientation, and which also provides rapid andcomplete CT baggage scanning so that the system 100 reliably scans thebags at a relatively high rate with a high probability of detectingtarget material. The system 100 includes a conveyor system 110 forcontinuously conveying baggage or luggage 112 in a direction indicatedby arrow 114 through a central aperture of a CT scanning system 120. Theconveyor system includes motor driven belts for supporting the baggage.Conveyer system 110 is illustrated as including a plurality ofindividual conveyor sections 122; however, other forms of conveyorsystems may be used.

The CT scanning system 120 includes an annular shaped rotating platform,or disk, 124 disposed within a gantry support 125 for rotation about arotation axis 127 (shown in FIG. 3) that is preferably parallel to thedirection of travel 114 of the baggage 112. Disk 124 is driven aboutrotation axis 127 by any suitable drive mechanism, such as a belt 116and motor drive system 118, or other suitable drive mechanism, such asthe one described in U.S. Pat. No. 5,473,657 issued Dec. 5, 1995 toGilbert McKenna, entitled "X-ray Tomographic Scanning System," (AttorneyDocket No. ANA-30CON) which is assigned to the present assignee andwhich is incorporated herein in its entirety by reference. Rotatingplatform 124 defines a central aperture 126 through which conveyorsystem 110 transports the baggage 112.

The system 120 includes an X-ray tube 128 and a detector array 130 whichare disposed on diametrically opposite sides of the platform 124. Thedetector array 130 can be a two-dimensional array such as the arraydescribed in a copending U.S. Patent Application entitled, "AreaDetector Array for Computed Tomography Scanning System," (AttorneyDocket No. ANA-137) filed on even date herewith, of common assignee, andincorporated herein in its entirety by reference. The system 120 furtherincludes a data acquisition system (DAS) 134 for receiving andprocessing signals generated by detector array 130, and an X-ray tubecontrol system 136 for supplying power to, and otherwise controlling theoperation of, X-ray tube 128. The system 120 is also preferably providedwith a computerized system (not shown) for processing the output of thedata acquisition system 134 and for generating the necessary signals foroperating and controlling the system 120. The computerized system canalso include a monitor for displaying information including generatedimages. The X-ray tube control system 136 can be a dual energy X-raytube control system such as the dual energy X-ray tube control systemdescribed in the above-referenced U.S. patent application Ser. No.08/671,202 since dual energy X-ray techniques for energy-selectivereconstruction of X-ray CT images are particularly useful in indicatinga material's atomic number in addition to indicating the material'sdensity, although it is not intended that the present invention belimited to this type of control system. System 120 also includes shields138, which may be fabricated from lead, for example, for preventingradiation from propagating beyond gantry 125.

In one embodiment, the X-ray tube 128 generates a pyramidically shapedbeam, often referred to as a "cone beam," 132 of X-rays that passthrough a three dimensional imaging field, through which baggage 112 istransported by conveying system 110. After passing through the baggagedisposed in the imaging field, cone beam 132 is received by detectorarray 130 which in turn generates signals representative of thedensities of exposed portions of baggage 112. The beam therefore definesa scanning volume of space. Platform 124 rotates about its rotation axis127, thereby transporting X-ray source 128 and detector array 130 incircular trajectories about baggage 112 as the baggage is continuouslytransported through central aperture 126 by conveyor system 110 so as togenerate a plurality of projections at a corresponding plurality ofprojection angles.

In a well known manner, signals from the detector array 130 can beinitially acquired by data acquisition system 134, and subsequentlyprocessed by a computerized system (not shown) using CT scanning signalprocessing techniques. The processed data can be displayed on a monitor,and/or can also be further analyzed by the computerized system todetermine the presence of a suspected material. For example, the datacan be received to determine whether the data suggests the presence ofmaterial having the density (and when a dual energy system is used,molecular weight) of sheet explosives. If such data are present,suitable means can be provided for indicating the detection of suchmaterial to the operator or monitor of the system, for example, byproviding an indication on the screen of a monitor 140, by sounding anaudible or visual alarm, and/or by providing an automatic ejectiondevice for removing the suspect bag from the conveyor for furtherinspection, or by stopping the conveyor so that the suspect bag can beinspected and/or removed.

As stated above, detector array 130 can be a two-dimensional array ofdetectors capable of providing scan data in both the directions of theX- and Y-axes, as well as in the Z-axis direction. During each measuringinterval, the plurality of detector rows of the array 130 generate datafrom a corresponding plurality of projections and thereby simultaneouslyscan a volumetric region of baggage 112. The dimension and number of thedetector rows are preferably selected as a function of the desiredresolution and throughput of the scanner, which in turn is a function ofthe rotation rate of rotating platform 124 and the speed of conveyingsystem 110. These parameters are preferably selected so that in the timerequired for a single complete rotation of platform 124, conveyingsystem 110 advances the baggage 112 just enough so that the volumetricregion scanned by detector array 130 during one revolution of theplatform is contiguous and non-overlapping with (or partiallyoverlapping with) the volumetric region scanned by detector array 130during the next revolution of the platform.

Conveying system 110 continuously transports a baggage item 112 throughCT scanning system 120, preferably at constant speed, while platform 124continuously rotates at a constant rotational rate around the baggageitems as they pass through. In this manner, system 120 performs ahelical volumetric CT scan of the entire baggage item. Baggage scanningassembly 100 preferably uses at least some of the data provided by thearray 130 and a helical reconstruction algorithm to generate avolumetric CT representation of the entire baggage item as it passesthrough the system. In one embodiment, the system 100 performs anutating slice reconstruction (NSR) on the data as described incopending U.S. patent application Ser. No. 08/831,558, filed on Apr. 10,1997, entitled, "Nutating Slice CT Image Reconstruction Apparatus andMethod," (Attorney Docket No. ANA-118) of common assignee, andincorporated herein by reference. The system 100 thus provides acomplete CT scan of each bag, rather than only providing CT scanning ofselected portions of baggage items, without the need for a pre-screeningdevice. The system 100 also provides rapid scanning sincetwo-dimensional detector array 130 allows the system 100 tosimultaneously scan a relatively large portion of each baggage item witheach revolution of the platform 124.

FIG. 4 contains a mechanical/electrical block diagram of one embodimentof the baggage scanning system 100 of the invention. The mechanicalgantry of the scanner 100 includes two major components, the disk 124and the frame (not shown). The disk 124 is the rotational element whichcarries the X-ray assembly, the detector assembly 130, the dataacquisition system (DAS) 134, a high-voltage power supply and portionsof the monitor/control assembly, the power supply assembly and the datalink assembly. The frame supports the entire system 100, including thebaggage handling conveyor system 110. The disk 124 is mechanicallyconnected to the frame via a duplex angular contact ball bearingcartridge. The disk 124 can be rotated at a constant rate by a beltwhich can be driven by a DC servomotor 505. The gantry also containsX-ray shielding on the disk and frame assemblies.

In one embodiment, the baggage conveyor system 110 includes a singlebelt driven at a constant rate to meet specified throughputrequirements, which, in one embodiment, include a requirement that 675bags per hour be processed. The belt can be driven by a high-torque,low-speed assembly to provide a constant speed under changing loadconditions. A low-attenuation carbon graphite epoxy material can be usedfor the portion of the conveyor bed in the X-ray. The total length ofthe conveyor is designed to accommodate three average length bags. Atunnel is used around the conveyor to meet the appropriate safetyrequirement of a cabinet X-ray system.

In one embodiment, input power of 208 volts, 3-phase, 30 amps servicesas the main supply which can provide power for the entire system. Thisinput power can be supplied by the airport at which the system isinstalled. Power is transferred from the frame through a series of framebrushes which make continuous contact with the metal rings mounted tothe disk 124. The low-voltage power supply 501 on the disk 124 providespower for the DAS 134, the X-ray cooling system and the variousmonitor/control computers and electronics. A low-voltage power supply onthe frame provides power for the reconstruction computer and the variousmonitor/control electronics. The conveyor motor 503, the gantry motor505, the high-voltage power supply and the X-ray coolant pump can all besupplied power directly from the main supply.

The high-voltage power supply provides power to the X-ray tube 128. Thesupply can provide a dual voltage across the cathode/anode which can bemodulated at 540 Hz. The driving waveform can be in the form of a sinewave. This supply can also provide X-ray filament power. The supplycurrent can be held approximately constant for both voltages.

The X-ray assembly includes a bipolar, fixed-anode X-ray tube 128, aheat exchanging system 507, a collimator 509, shielding, an X-ray sensorand an alignment/mounting plate. The collimator can provide an X-raycone beam of 61° fan angle by 6° spread. The heat exchanging system 507includes apump, radiator, fan and plumbing. The heat transfer liquid canbe a high-dielectric oil. An alignment plate can be used for mountingthe tube 128 to the disk 124 to reduce the field replacement complexityand time. An X-ray sensor can be included to provide X-ray intensityfeedback.

The dual-energy X-rays strike the baggage, and some portion of theX-rays pass through and strike the detector assembly 130. The detectorassembly 130 can be made up of scintillators, photodiodes, mountingsubstrates, anti-scatter plates and a mechanical mounting spine. A spineheater with temperature sensors 521 can also be included. The detectorassembly 130 performs an analog conversion from X-ray to visible photonsand then to electrical current. The anti-scatter plates can be made ofhigh-atomic-number material and are angled at the X-ray source to reducethe amount of scattered radiation that strikes the scintillators. Thescintillators are made from cadmium tungstate crystal which is thickenough to almost completely absorb all of the X-rays. The scintillatorsconvert the X-rays into visible photons. The crystal can be surroundedon all sides except the bottom by optically reflective material. Thus,the visible photons can pass out of the bottom of the crystal. Thephotodiodes can be connected to the bottom of the crystal by means of anoptically transmissive adhesive. The photodiodes emit a current whichdecreases logarithmically with the bag's X-ray attenuation. Thephotodiodes can be attached to a ceramic substrate which can be sized tofit several detectors. This electrical substrate can be wire bonded andepoxied to a flexprint which contains a connector which mounts to theDAS 134. Each detector substrate can then be mechanically attached to amounting spine that has the fan beam radius and projects in theZ-direction. This spine can then be rigidly secured to the disk 124.

The DAS 134 can sample the detector currents, multiplex the amplifiedvoltages to a set of 16-bit analog-to-digital converters and multiplexthe digital outputs to the non-contact serial data link 511. The DAS 134can be triggered by the angular position of the disk 124.

The non-contact links 511 and 513 transfer the high-speed digital DASdata to the image reconstruction processor 515 and low-speedmonitor/control signals back and forth between the disk and framecontrol computers. The data link 511 can be based upon an RF transmitterand receiver. The transfer protocol can be TAXI™ which is capable of upto 350 Mbits/sec. The control link 513 can be based on wireless LANtechnology, which can include identical PCMCIA cards mounted in both theframe and disk computers. The cards can have both a transmitter andreceiver electronics and can emulate a standard Ethernet card. Apoint-to-point network is therefore established for the low-speedmonitor and control communication.

The image reconstructor converts the digital line integrals from the DAS134 into a set of two-dimensional images of bag slices for both the highand low energies. The CT reconstruction can be performed via ahelical-cone-beam solution. The reconstructor can include embeddedsoftware, a high-speed DAS port, an array processor, a DSP-basedconvolver, an ASIC-based backprojector, image memory, UART control port,and a SCSI output port for image data. The array processor can performdata corrections and interpolation. The reconstructor can be self-hostedand can tag images based upon the baggage information received over theUART interface to the frame computer.

The monitor and control system can be a PC-based embedded controlsystem. All subsystems can be monitored for key health and statusinformation. This system can also control both motion systems, can sensebaggage information, can control the environment, e.g., temperature,humidity, etc., can sense angular position of the disk 124 and cantrigger the DAS and HVPS. This system can also have a video and keyboardinterface for engineering diagnostics and control. Additionally, acontrol panel can be included for field service.

The CT baggage scanner of the invention includes the ability to tailorthe image reconstruction window to the bag being scanned in order toimprove the baggage throughput of the system. Before reconstructing animage of the bag, the system of the invention can distinguish pixels tobe reconstructed to generate the image of the object from pixels whichare not to be reconstructed. The reconstructed pixels are those relatedto the density of the bag being scanned. Pixels that are unrelated tothe bag are not reconstructed and are therefore effectively discarded.The discarded pixels include pixels for the region under the conveyor aswell as regions next to and above the bag. By omitting a substantialnumber of pixels from the reconstruction process, processing time isreduced and, as a result, baggage throughput is increased.

FIG. 5 is a schematic pictorial diagram of the field of view 350 of thescanner, used to illustrate the adaptive reconstruction window of theinvention. The field of view 350 is shown to include the conveyor 1 10on which is located a bag 112 having a height h and a width w. The fieldof view also includes a region 351 below the conveyor 110, a region 352above the bag 112 and regions 353 on opposite sides of the bag 112.These regions 351, 352 and 353 are scanned by the system of theinvention, and scan data is acquired for them. However, since the bag112 is not located in these regions, image pixels for these regionscontribute no information concerning the bag and are therefore discardedfrom the image reconstruction process for the bag.

FIG. 6 is a schematic plot showing the field of view 350 of the scannersuperimposed on an x,y Cartesian coordinate system. An image of the bag112 being scanned can be regarded as being generated from a rectangulararray of pixels 357. The bag can be regarded as being N pixels wide andM pixels high, and each pixel can be considered as having equal heightand width dimensions of p, typically measured in millimeters. Hence, thewidth w of the bag 112 is given by w=Np, and the height h of the bag 112is given by h=Mp. By determining the actual height and width of a bagbeing scanned, the number of pixels 357 to be reconstructed, given byN×M, can be calculated. The location of the pixels to be reconstructedcan also be determined by locating the center of the bag, located atcoordinate x_(o),y_(o).

The height h, width w and center x_(o),y_(o) are determined by locatingthe boundaries of the bag 112. As shown in FIG. 6, the bottom and top ofthe bag are given by coordinates y₁ and Y₂, respectively, and the leftand right edges of the bag are identified by coordinates x₁, x₂,respectively. The center 354 is identified by x_(o), y_(o), where x_(o)=(x₂ -x₁)/2 and y_(o) =(y₂ -y₁)/2. The height h is given by h=y₂ -y₁,and the width w is given by w=x₂ -x₁. The number N of pixel columns andthe number M of pixel rows are then determined from the width w andheight h, respectively, using the known pixel dimension p.

With the height, width and location of the bag determined, the totalnumber of pixels N×M can be calculated as described above. In oneembodiment, this total number of pixels is reconstructed to produce animage of the bag. In another embodiment, to ensure acceptable andcontrollable baggage throughput, pixel reconstruction is limited to apreset maximum number of pixels to be reconstructed. The desired systembaggage throughput is used to determine this maximum number of pixels tobe reconstructed for every bag. In one embodiment, this maximum numberof pixels is set at 25,000. The total number of pixels N×M required toreconstruct an image of the bag is compared to this preset pixel limit.If N×M is less than the limit, then the N×M pixels are reconstructed.However, if N×M exceeds the limit, then the reconstruction window usedfor the particular bag is fit to the best possible pixel window thatcomplies with the limit. Reconstruction is then performed on the limitednumber of pixels.

As described above, the height, width, center location and pixeldimensions N and M are derived from the boundary locations of the bagwithin the field of view of the scanner. The boundaries can be locatedby any of several possible methods. In one embodiment, the scan dataitself is analyzed to locate the boundary locations x₁, x₂, y₁, and y₂.This can be done by examining parallel projection data generated fromthe scan data. In another embodiment, a separate sensor on the scanningmachine is used to detect the bag boundaries.

FIG. 7 contains a simplified schematic block diagram of one embodimentof the baggage scanning system 100 of the invention which uses separatesensors to determine the boundaries of a bag. The system 100 shown inFIG. 7 includes the CT scanner 120 and conveyor system 110 which carriesbags 112 through the scanner 120. One or more sensors 360 which can bemounted to the scanner 120 are used to detect the boundaries of the bag112 as it enters the scanner 120. The sensors 360 can include aconfiguration of one or more lasers and photodetectors to detectboundaries. Alternatively, the sensors 360 can include infrareddetectors and/or a combination of light-emitting diodes andphotodetectors to sense the boundaries. Alternatively, the sensors 360can include high-frequency ultra sound transducers used as range findersto detect the boundaries of the bag 112.

The sensor outputs are routed to a sensor output processing circuit 370which processes the outputs to determine the boundaries of the bag.Detector signals generated by detectors in the scanner 120 are forwardedto a data acquisition system (DAS) 134 which processes the detectoroutputs and generates corresponding signals and forwards them to aprocessing system 364. The processing system 364 also receives outputsfrom the sensor processing circuit 370 which identify the boundaries ofthe bag. The processing system 364 generates image data from thedetector data in order to generate an image of the bag 112.

The CT baggage scanning system of the invention also provides forcalibrating the system such that compensation can be made for variationsin detector responses from detector to detector. This calibration isperformed by a calibration or "air" scan of the field of view of thesystem. In a conventional medical CT system, when the air scan isperformed, all obstructions, such as the patient table, are removed fromthe field of view. A complete scan of the field of view is thenperformed and data acquired by the detectors are analyzed. In a baggagescanner, such as the scanner of the invention, obstructions in the fieldof view, such as the conveyor system, are not as readily removable toallow for an air scan. The system of the invention allows air scans tobe performed without removing obstructions from the field of view.

FIGS. 8A and 8B illustrate the conventional air scan. FIG. 8Aschematically illustrates the configuration of the conventional CTscanner. The scanner includes a source 204 and detector array 202 whichsimultaneously rotate about a center of rotation 203 in acounterclockwise direction as illustrated by arrow 206. The source 204and detector array 202 can be regarded as rotating through a series ofviews v about the center of rotation. At each view v, a series ofsamples s corresponding to the detectors in the array 202 is acquired.Typically, the field of view includes a circular window 200 which issemi-transparent to x-rays from the source 204.

FIG. 8B is a schematic plot of the data signal obtained for a singledetector or sample s over the entire range of views v. As shown, becauseobstructions are removed from the field of view and because the window200 is circular and therefore presents a constant density over allviews, the data signal received for each detector over all views is, ingeneral, constant. In a conventional air scan, a data set such as theone plotted in FIG. 8B is obtained for each detector in the array 202. Acalibration factor is computed for each detector such that, when thefactor is applied to the detectors, their responses are all equal.

In this ideal conventional situation, since the data signals for eachdetector do not vary with view, the calibration factor computed for eachdetector is view-independent. That is, the calibration factor applied tothe data gathered by a detector is the same for every view at which thedetector gathers data. As a result, each detector is associated withonly a single calibration factor. In reality, however, the response ofthe detector is not exactly independent of view. Because ofgravitational and other effects, the line plotted in FIG. 8B is notactually flat. Accordingly, the calibration factor is dependent uponview. Therefore, for each detector, a calibration factor is computed foreach view, resulting in a large calibration look-up table, whichconsumes considerable memory space.

FIG. 9A schematically illustrates the scanning configuration of thebaggage scanner system 100 of the present invention. As shown in FIG.9A, the configuration differs from the conventional configuration shownin FIG. 8A. In the baggage scanner 100, the conveyor system 110 ispresent in the field of view and remains as an obstruction in the fieldof view during an air calibration scan. Also, the machine aperture 126is not circular as is the aperture in the conventional machine. Themachine of the invention can also include a window 220 which is notcircular, in contrast to the circular window 200 in the conventionalscanning system. These factors combine to produce a detector responseduring an air scan which is dependent upon view, as shown in FIG. 9B,which is a plot of the data signal generated by a single detector orsample s over all views v during an air calibration scan of the field ofview of the baggage scanner of the invention. It will be understood thatthe shape of the curve shown in FIG. 9B is merely illustrative of anuneven view-dependent detector response and is not intended toaccurately represent the actual response of any detector.

To perform an effective air calibration scan, it is desirable to useonly views associated with rays that do not pass through an obstruction.In the present invention, the data illustrated by FIG. 9B for eachdetector are analyzed and data associated with unobstructed rays areselected for use in computing the calibration adjustment for thedetector. In one embodiment, this is accomplished by setting a detectorsignal threshold T and processing data values according to where theyfall with respect to T. The threshold T can be set such that data valuesabove the threshold T can be assumed to be generated by unobstructedrays passing through the field of view. The calibration factor can thenbe computed using only those data values that exceed the threshold T.

For example, as shown in FIG. 9B, two ranges of views 223 and 225generate data values above the threshold T. It is these views that areassumed to be generated from unobstructed ray paths through the field ofview of the given detector sample s. Therefore, only the data valuesgenerated in these two ranges of views are used to compute thecalibration factor for this detector. In one embodiment, the data valuesabove the threshold T are averaged and the average value is used todetermine the calibration factor for that detector or sample s. Thisresults in a single calibration factor being determined for thedetector, and this single calibration factor can be used for all datagathered by the detector over all views. That is, the calibration isview-independent.

Hence, even though the response of the detector, due to obstructions inthe field of view, is dependent upon view, the invention distinguishesobstructed views from unobstructed views, and as a result, generates aview-independent calibration for the detector. This saves substantialmemory and computation in both storing the calibration factors and inadjusting the data values during subsequent scans of actual objects.

During actual scans, scan data are normalized to account for thecalibration factor computed for each detector during the air calibrationscan. For each projection over every view and sample, a normalized valuecan be computed according to P_(vs) =1n(A_(vs) /D_(vs)), where P_(vs) isthe projection data at a particular view v and detector sample s, A_(vs)is the calibration factor obtained during calibration at the view v andsample s, and D_(vs) is the actual data gathered at view v by detectorsample s. It should be noted that, as described above, A_(vs) is thesame over all v, in one embodiment.

The threshold T can be computed by several different approaches. In oneapproach, the maximum data value, indicated by reference numeral 227 inFIG. 9B, can be multiplied by a constant factor to compute the thresholdT. Preferably, the factor is a fraction slightly less than 1, e.g.,0.95. Such a relatively high threshhold is selected to provide a highlevel of confidence that only data values associated with unobstructedrays are used in the computation of the calibration factor.

The present invention also includes an apparatus and method fordetecting the shape of an object, particularly sheet-shaped objects, inthe three-dimensional CT image data for an object. It is assumed thatthe object is defined by boundaries or an outside surface and that eachpixel in the image is representative of density of the object at thatpixel. FIG. 10 is a schematic illustration of a three-dimensional CTimage of an object 300. For ease of illustration, the object 300 isshown in two dimensions. However, it will be understood that theinvention applies to three-dimensional objects.

In accordance with the invention, a series of points 302 along thesurface of the object 300 is identified and analyzed. Given the pixelsthat define the surface in three-dimensional space, a surface normalvector N at any given location can be determined such as by computingthe gradients of the surface. At each point 302, a surface normal vectorN is identified. For each normal N, a normal line 304 is projected backinto the object 300. A series of points 306 along the normal line 304are then identified, and a density value from the CT data for the objectis assigned to each of the data points 306. To determine the density ateach point 306 along the normal line 304, interpolation between pixelvalues can be used. A maximum thickness T_(MAX) is set to define themaximum distance along the normal line 304 that data points 306 will becomputed. The maximum thickness T_(MAX) is chosen to be larger than themaximum expected thickness of a sheet.

The density distribution of the points 306 along the normal line 304 isdetermined and analyzed. FIG. 11 shows sample distributions for twocases of data points 306 along a surface normal 304. In onedistribution, labeled 310, the density p is relatively constant into theobject 300 out to and beyond the maximum measured thickness T_(MAX). Inthe distribution labeled 312, the density function rolls off at somedistance T_(R). This roll-off distance T_(R) is indicative of thethickness of the object 300 at the associated surface normal line 304.Hence, at that particular surface normal N, the object 300 is relativelythin. Where there is no roll-off, as shown by curve 310, the object atthe associated point 302 is relatively thick. In fact, it is at least asthick as the preset maximum thickness T_(MAX).

This process is repeated for a plurality of locations 302 along thesurface of the object 300, and a distribution such as that shown in FIG.11 is analyzed such that a value for T_(R) is assigned to each surfacenormal N. The value of T_(R) can be the point along the curve at whichthe density rolls off, such as shown in FIG. 11. Alternatively, thevalue of T_(R) can be computed as the mean of the curve 312 for theassociated surface normal N. In cases where the distribution does notroll off, such as is shown by curve 310, T_(R) is set to the maximumthickness T_(MAX).

A histogram can then be generated for all of the T_(R) values, as shown,for example, in FIGS. 12A and 12B. FIG. 12A shows a histogram in whichthe object 300 can be concluded to be a sheet. In this example, T_(MAX)is set to 10 mm and a peak in the histogram occurs at about 4 mm. Thisindicates that a large portion of the roll-off values T_(R) for points302 along the surface are at 4 mm. With such a large portion ofthicknesses being substantially below the maximum T_(MAX), it can beconcluded that the object 300 is a sheet.

To analyze the histogram, a threshold T can be set. If a peak in thecurve exceeds the threshold, as shown in FIG. 12A, then it can beconcluded that the peak indicates a sheet having a thickness at thelocation of the peak along the horizontal axis, e.g., 4 mm. The smallrise in the curve at T_(MAX) (10 mm) indicates that a substantial numberof measurements showed thicknesses beyond the maximum thickness T_(MAX).This is mostly due to measurements taken along the thin edge of thesheet which tend to indicate high densities extending deep into theobject. But, the peak at 4 mm is so much higher than the rise at 10 mmthat, statistically, a sheet is indicated.

FIG. 12B shows a histogram produced for the case in which the object 300is not a sheet. As shown in FIG. 12B, there is a peak in the histogramat the maximum thickness T_(MAX) (10 mm), which indicates that a largeportion of the measurements show a thickness which exceeds the maximumthickness expected for a sheet. Hence, it can be concluded that theobject 300 is not a sheet. The method of truncating the distribution atT_(MAX) decreases processing time and eliminates computational problemsassociated with the surface normals along the edge of a sheet.

In one embodiment, the statistical analysis can be performedautomatically on a processor. The histogram can be automaticallysearched for peaks. The location and shape of a peak can determine ifthe object is a sheet by comparison with an existing dataset or adatabase.

The CT scanning system of the invention also has the ability tocompensate for temperature-dependent "dark current" detector offsetcurrents or, simply, "offsets." A dark current is a current generated bya detector with the x-ray source turned off, i.e., while the detector isnot receiving x-rays. This residual or quiescent current can causeinaccuracies in the scan data obtained during normal object scanning.Adding to the inaccuracies introduced by dark currents is the fact thatthey are also dependent upon temperature and may vary from detector todetector. An adjustment is calculated to compensate for the dark currentoffsets to reduce these inaccuracies.

In the present invention, temperature-dependent offsets can becalculated for each detector. In one embodiment, a calibration procedureis performed before actual scanning to characterize the temperaturedependence of the offsets. During the calibration procedure, currentsfrom the detectors are measured while the temperature of the detectorsis cycled. Plural data points, i.e., offset versus temperature, areobtained for plural detectors. In one embodiment, data points areobtained for each detector. In another embodiment, only a subset of allthe detectors is used. An average of the offsets can then be used foreach detector.

After the temperature dependence characterization is obtained, actualscanning can be performed. A temperature sensor mounted in proximity tothe detectors senses the temperature of the detectors as scanning isperformed. Comparing the present temperature to the stored temperaturedependence function identifies an offset to be applied to the dataobtained by the scanning detectors. In one embodiment, periodic scanswith the X-ray source turned off are performed between bags. Theseperiodic dark current scans are performed at a known temperature T₁.Later, when actual scans arc performed, the temperature T₂ is sensed.The difference between the dark current scan temperature T₁ and thepresent scanning temperature T₂ is applied to the stored temperaturedependence function obtained during the calibration procedure toidentify an appropriate offset to be applied to the detector data.

To illustrate the temperature dependent offset function of theinvention, let x be the temperature dependent offset of a single channelunder consideration. The temperature dependence of the offset is givenby the following Taylor series expansion:

    x(T)≈α.sub.0 +α.sub.1 T+α.sub.2 T.sup.2,(1)

where T is the temperature of the channel and α_(p) I=0,1,2, areconstants.

The values of α_(i) are determined using the calibration procedure.Calibration of the temperature dependence of the offsets is requiredinfrequently, perhaps only annually, or when a defective detectorelement is replaced. During the calibration, the temperature of thedetector is varied so that the offset can be measured at a number oftemperatures representative of the typical operating range of thescanner. Let x_(i) be the offsets measured at temperatures T_(i), whereI=0, 1, . . . , N-1 and N is the number of measurements. A typical valueof N is five. The values of α_(i) are estimated using least squareerrors as follows:

Let C be a matrix of α_(i) as in: ##EQU1## The matrix C is found using

    C=A.sup.-1 B                                               (3)

where ##EQU2##

In the course of a scanner's operation, a detector's offset isperiodically measured. During typical operation, the offset is measuredhourly. Let T₁ be the temperature at which the offset is measured. Alsolet x(T₁) be the offset at this first temperature. At some time later,during scanning, the detector will be at temperature T₂. The value ofthe offset at this second temperature, x(T₂), follows from (1) as

    x(T.sub.2)≈x(T.sub.1)+α.sub.1 (T.sub.2 -T.sub.1)+α.sub.2 (T.sub.2.sup.2 -T.sub.1.sup.2).   (7)

In practice, there are D detectors and the detector temperatures aremeasured for only a small subset of the total number D of detectors. LetN be the number of temperature readings, where a typical value might befive. The temperature readings are taken by temperature sensors such as,for example, the five temperature sensors 521 shown in FIG. 4. Let d_(i)be the detectors at which the temperatures are measured, where I=0, 1, .. . , N-1, where, again, N is the number of temperature readings. LetT_(di) be the measured temperatures at detectors d_(i). The temperatureT_(j) at detectors where j=0, 1, . . . , D-1, can be estimated byfitting a second-order polynomial through the N readings. The parametricform for the temperature dependence is

    T.sub.j ≈β.sub.0 +β.sub.1 j+β.sub.2 j.sup.2,(8)

where β_(i) are constants that can be determined via least squareserrors as follows:

Let ##EQU3## The matrix F is found using

    F=D.sup.-1 E                                               (10)

where ##EQU4## The temperatures at detectors d_(i) can be measured usingthermocouples and resistive temperature detectors (RTD).

While this invention has been particularly shown and described withreferences to preferred embodiments thereof, it will be understood bythose skilled in the art that various changes in form and details may bemade therein without departing from the spirit and scope of theinvention as defined by the following claims.

What is claimed is:
 1. A method of performing an air calibration of acomputed tomography (CT) scanning system, said CT scanning systemincluding a source of radiation and an array of detectors for receivingthe radiation propagating through a field of view of the CT scanningsystem and generating detector signals related to radiation received bythe detectors, at least one of the source of radiation and the array ofdetectors being rotatable about a rotation axis to generate scan datavalues from the detector signals for the field of view at a plurality ofview angles about the rotation axis, said method comprising:performing acalibration scan of the field of view to generate calibration scan datafor the field of view, the calibration scan being performed with anobject in the field of view; for each of a plurality of detectors in thearray of detectors, setting an associated calibration threshold for thecalibration scan data values acquired for the detector at the pluralityof view angles; for each of the plurality of detectors, comparingcalibration scan data values for the detector to the associatedcalibration threshold; identifying calibration scan data values thatexceed the associated calibration threshold as calibration scan datavalues acquired for radiation that did not pass through the object;selecting the calibration scan data values that exceed the associatedcalibration threshold; and for each of the plurality of detectors, usingthe selected calibration scan data values, computing a detector aircalibration value, said detector air calibration value being used duringsubsequent scans to adjust scan data acquired by the detector.
 2. Themethod of claim 1 wherein computing the detector air calibration valuefor each detector comprises averaging the selected calibration scan datavalues for the detector.
 3. The method of claim 1 wherein the object inthe field of view comprises a portion of a conveyor for moving objectsthrough the CT scanning system.
 4. The method of claim 1 wherein anassociated calibration threshold is set for each detector in the array.5. The CT scanning system of claim 2 wherein the object in the field ofview comprises a portion of a conveyor for moving objects through the CTscanning system.
 6. The CT scanning system of claim 2 wherein anassociated calibration threshold is set for each detector in the array.7. A computed tomography (CT) scanning system comprising:a source ofradiation for directing radiation through a field of view of the CTscanning system; an array of detectors for receiving the radiation andgenerating detector signals related to radiation received by thedetectors, at least one of the source of radiation and the array ofdetectors being rotatable about a rotation axis to generate scan datavalues from the detector signals for the field of view at a plurality ofview angles about the rotation axis; means for performing a calibrationscan of the field of view to generate calibration scan data for thefield of view, the calibration scan being performed with an object inthe field of view; means for setting, for each of a plurality ofdetectors in the array of detectors, an associated calibration thresholdfor the calibration scan data values acquired for the detector at theplurality of view angles; means for comparing, for each of the pluralityof detectors, calibration scan data values for the detector to theassociated calibration threshold; means for identifying calibration scandata values that exceed the associated calibration threshold ascalibration scan data values acquired for radiation that did not passthrough the object; means for selecting calibration scan data valuesthat exceed the associated calibration threshold; and means forcomputing, using the selected calibration scan data values, a detectorair calibration value for each of the plurality of detectors, saiddetector air calibration value being used during subsequent scans toadjust scan data acquired by the detector.
 8. The CT scanning system ofclaim 7 wherein the means for computing the detector air calibrationvalue for each detector comprises means for averaging the selectedcalibration scan data values for the detector.
 9. An air calibrationsystem for a computed tomography (CT) scanning system, the CT scanningsystem having a source of radiation for directing radiation through afield of view of the CT scanning system and an array of detectors forreceiving the radiation and generating detector signals related toradiation received by the detectors, at least one of the source ofradiation and the array of detectors being rotatable about a rotationaxis to generate scan data values from the detector signals for thefield of view at a plurality of view angles about the rotation axis,said calibration system comprising:means for performing a calibrationscan of the field of view to generate calibration scan data for thefield of view, the calibration scan being performed with an object inthe field of view; means for setting, for each of a plurality ofdetectors in the array of detectors, an associated calibration thresholdfor the calibration scan data values acquired for the detector at theplurality of view angles; means for comparing, for each of the pluralityof detectors, calibration scan data values for the detector to theassociated calibration threshold; means for identifying calibration scandata values that exceed the associated calibration threshold ascalibration scan data values acquired for radiation that did not passthrough the object; means for selecting calibration scan data valuesthat exceed the associated calibration threshold; and means forcomputing, using the selected calibration scan data values, a detectorair calibration value for each of the plurality of detectors, saiddetector air calibration value being used during subsequent scans toadjust scan data acquired by the detector.
 10. The calibration system ofclaim 9 wherein an associated calibration threshold is set for eachdetector in the array.
 11. The calibration system of claim 9 wherein themeans for computing the detector air calibration value for each detectorcomprises means for averaging the selected calibration scan data valuesfor the detector.